The present invention relates to an electron beam apparatus for inspecting a pattern or the like formed on a surface of a sample, and more particularly to an electron beam apparatus for reliably inspecting a wafer or a mask having a pattern with a minimum line width of 0.1 μm or less at a high throughput.
Conventionally, an electron gun using a Schottky cathode has been employed for inspecting a sample for defects on a pattern having a size of 0.1 μm or less. For example, Hiroyuki Shinada et al. “High-Speed Large-Current SEM Optical System for Inspecting Wafers,” Proceedings of LSI testing symposium/2000 (Nov. 9-10, 2000), pp. 151-156 discloses a SEM using a Schottky cathode electron source, which is capable of effectively inspecting a wafer for defects even if the wafer has electric defects such as circuit-short, circuit opening and the like occurring during a forming step and a wiring step in underlying layers of the wafer.
An apparatus and method for inspecting a sample for defects is known in which a plurality of primary electron beams (or charged particle beams) are generated around a single optical axis. For example, U.S. Pat. No. 5,892,224 discloses an apparatus for inspecting a sample such as a mask, reticle or the like, in which the sample is simultaneously irradiated with a plurality of primary electron beams around one optical axis and a plurality of secondary electrons emitted from the sample are detected. Another apparatus for inspecting a sample has been proposed in which a plurality of electron guns are employed to irradiate a plurality of electron beams that are guided to the sample through a plurality of optical systems associated therewith.
It is to be noted that the entire contents of Hiroyuki Shinada et al. and U.S. Pat. No. 5,892,225 are incorporated herein by this description.
As described above, the SEM optical system in Shinada et al. employs a Schottky cathode as an electron gun. However, such a Schottky cathode based electron gun is a problematic in that it generates a large noise. To overcome this problem of providing a signal with a high S/N ratio, a sample must be irradiated with an electron beam of high irradiation, which could however damage the sample.
The electron beam apparatus disclosed in U.S. Pat. No. 5,892,224 can reduce an inspection time by virtue of the use of a plurality of primary electron beams. However, this apparatus has only one optical axis and, as a result, the primary electron beams are formed at respective positions away from the optical axis, resulting in a relatively large aberration. For this reason, when the respective electron beams are narrowed down to a certain size, resulting beam currents are smaller than a beam current of an electron beam which lies on the optical axis. Also, the electron beam apparatus requires a relatively complicated optical system for guiding and detecting secondary electrons, and which makes it difficult to detect the secondary electrons at a rate approaching 100%. Further, difficulties are experienced in using the apparatus to adjust the primary electron beams independently of one another, and as a consequence, it is difficult to match the magnitudes of the primary electron beams.
In an electron beam apparatus which comprises a plurality of electron guns for simultaneously emitting a plurality of primary electron beams, the use of thermal electron emission cathodes for the electron guns could lead to excessive heat being generated by the electron guns. If the electron guns are thermaled to such an extent to damage the stability of a system, each unit comprising primary and secondary optical systems encounters a problem of a lower degree of vacuum. Further, a thermaled or hotted electron emission cathode gives rise to a problem of a change in relative position between the thermal electron emission cathode and an anode.
A conventional exemplary electron beam apparatus forms an image comprised of pixels, each of which has dimensions or sizes equal to or smaller than the sizes of intended defects, and detects any defects from the formed image. For example, in detecting defects having a size of 0.05 μm or more on a pattern having a line width of 0.1 μm, the pixel size must be set to 0.05 μm or less. Also, in this event, if a clock frequency of 100 MHz (a scanning speed) is utilized to detect defects, a detection time period per cm2 is calculated as follows:102/{(5×10−5)2×100×106}=400 seconds≈0.111 hoursTherefore, in inspecting an area of 100 cm2 to detect defects thereon, 11.1 hours will be required even a simple calculation is employed.
While the foregoing example employs a clock frequency of 100 MHz, this clock frequency can be employed only in an electron beam apparatus which comprises a secondary electron beam detector having a response frequency sufficiently higher than 100 MHz.
For detecting defects of similar sizes to those of pixels, Shinada et al. teaches that an electron beam apparatus must provide an S/N ratio of 15 or higher where the noise is 3 s value. However, scanning is generally impossible at a clock frequency of 100 MHz or higher in a secondary electron beam detector, for example, when implemented by a combination of a scintillator and PMT, which does not provide a very high response frequency. Thus, an electron beam apparatus using such a secondary electron beam detector suffers from a problem in that it requires a long inspection time period and achieves a lower throughput, because it must scan a sample at a clock frequency lower than 100 MHz.